Optical encoders are used to monitor the motion of, for example, a shaft such as a crankshaft. Optical encoders can monitor the motion of a shaft in terms of position and/or number of revolutions of the shaft. Optical encoders typically use a codewheel attached to the shaft to modulate light as the shaft and the codewheel rotate. The light is modulated as it passes through a track on the codewheel that includes a pattern of transparent and opaque sections. As the light is modulated in response to the rotation of the codewheel, a stream of electrical signals is generated from a photodetector array that receives the modulated light. The electrical signals are used to determine the position and/or number of revolutions of the shaft.
Optical encoders that are used to determine absolute position typically have a separate track for each bit of resolution that is desired. For example, an encoder with 4-bit resolution would require four different tracks, with each track requiring its own photodetector. The tracks must be separated from each other by enough distance that light from the different tracks does not mix at the photodetectors. Optical encoders are employed in systems that desire high resolution with tight size limitations. Because the resolution of conventional optical encoders is a function of the number of tracks, what is needed is an optical encoder that can generate multi-bit resolution with a reduced number of tracks.